A rifle is a type of weapon that fires rounds. Typically, the rifle includes a trigger, a hammer, and a barrel. The round is positioned in the barrel, and when the trigger is pulled, the hammer fires the round through the barrel. One type of rifle is an M16-style rifle that features a gas-operated bolt and bolt carrier system as disclosed in U.S. Pat. No. 2,951,424, issued to Eugene M. Stoner on Sep. 6, 1960 and incorporated by reference herein in its entirety. The M16-style rifle is the weapon used by many military and civilian police forces today.
Typically, M16-style rifles include a lower receiver that houses the trigger and hammer, and an upper receiver that houses the barrel, bolt, and bolt carrier. The lower receiver and the upper receiver are detachably coupled together using a coupling mechanism, so that the rifle can be moved with relative ease between an assembled position suited for firing, and a disassembled position suited for cleaning the rifle and for repairing its internal parts. A conventional takedown system features front and rear pins slidably positioned in the lower receiver, and corresponding front and rear connector tabs extending from the upper receiver. A distance between the pins on the lower receiver corresponds to a distance between the connector tabs on the upper receiver, so that the pins can be pushed into and out of the connector tabs to selectively attach and detach the upper and lower receivers.
In some cases, the upper receiver can be substituted with a different upper receiver, such as a close quarters battle receiver (CQBR), which is a replacement upper receiver having a relatively shorter barrel. Like the conventional upper receiver, the CQBR has connector tabs that are separated by a distance corresponding to the distance between the pins on the lower receiver, so that the CQBR can be coupled to the lower receiver. In the field, the CQBR is employed during indoor operations or other operations in which a longer barrel rifle may be cumbersome to maneuver.
While the barrel length of the M16-style rifle can be changed with relative ease, the size of the rounds fired by the M16-style rifle cannot. Typically, the lower receiver has an integrally connected magazine well that is configured to receive a magazine, and the magazine is configured to hold the rounds. Because the magazine well can only be used with certain magazines, and each magazine can only be used with certain rounds, the size of the rounds that can be fired by the M16-style rifle is restricted.
In limited cases, the M16-style rifle may be reconfigured to fire rounds of other sizes, such as by manually rigging the magazine well to accept a non-standard magazine. Even with manual reconfiguration, however, most M16-style rifles cannot fire particularly large rounds because the magazine well is too small. In such cases, the lower receiver can be substituted with another lower receiver having an appropriately configured magazine well. However, substituting the lower receiver may be undesirable because the lower receiver of each M16-style rifle is tracked by the U.S. government, and requires a Federal Firearms License (FFL). Further, the substitute lower receiver may have pins that are separated by a distance that is different than the distance between the connecting tabs on the upper receiver. Therefore, when the lower receiver is substituted, it may be necessary to substitute the upper receiver with a different upper receiver having appropriately spaced connecting tabs.